The invention relates to a device for detecting the position of a moveably arranged magnet for generating a magnetic field through a wall made of ferromagnetic material, especially an actuating drive with a control element that supports the magnet and that can be moved behind a housing wall made of a ferromagnetically conductive material, whereby in front of the wall, there is a magnetic field sensor according to the generic part of claim 1.
Magnetic sensors serve for contact-free detection or measurement of physical quantities such as position, path, distance, speed or angle of rotation. In many applications, the sensor is controlled by a permanent magnet and then converts the position of this magnet relative to the sensor element into an electrical signal. Examples of such sensors are magnetic field sensors, for example, saturation core probes, GMR (Giant Magneto Resistive sensors), magneto-resistive sensors or Hall elements that are suited for control with permanent magnets as well as for detecting iron parts. Only the magnetic field component that is parallel to the sensor axis is effective in driving the position sensor.
EP 0,457,762 A describes an actuating drive with a control element that can be moved behind a housing wall, whereby a device for generating a magnetic field is attached to said control element and there is a magnetic field sensor in front of the housing wall of the actuating drive. The housing wall is made of a magnetically conductive material, whereby the field lines of the magnetic field in the housing wall form a main flux that is shielded relative to the front of the housing wall. In order to generate a magnetic secondary flux on the front of the housing wall, there is a magnetic conductor having two ends, whose first end is adjacent to the housing wall and whose second end delimits an air gap in which the magnetic field sensor is installed. Likewise, there can also be two magnetic conductors whose second ends are arranged opposite from each other, thus forming an air gap. The actuating drive is configured as a hydraulic high-pressure cylinder with a piston connected to a piston rod as the control element, whereby the cylinder wall forms the housing wall and the device for generating the magnetic field is configured as a permanent magnet. Likewise, in this publication, it was already suggested that it would be conceivable to arrange a Hall probe directly on the outside of the cylinder wall and then to only provide a magnetic conductor whose second end covers the back of the Hall probe, whereby an air gap relative to the cylinder wall is also provided.
The invention is based on the objective of improving an actuating drive in such a way that said actuating drive is capable of reliably detecting the position of the control element in a simple manner with simple means, even in the case of a magnetically shielding housing wall.
According to the invention, the objective is achieved in that a device for detecting a position of the class described above in that, when the magnet is moved relative to said wall, the field lines of the magnetic field build up a main flux which progresses within said wall, whereby said main flux forms a magnetic remanence which is retained after the magnet has passed by and it is directed along the movement axis of the magnet, in accordance with the polarity of the magnet, and builds up a leakage flux towards the front of the wall to the outside thereof, where the magnetic field sensor is positioned, whereby, outside of said housing wall, the leakage flux is directed in the opposite direction from the main flux, and the magnetic field sensor is a magnetic field sensor element that is located at a slight distance from the wall and that detects the field direction of the leakage flux and, when the magnet passes by, the leakage field is superposed by the magnetic field of the magnet, and the magnetic field sensor is capable of registering the change in the polarity of the magnetic field, and a switching signal is derived from the sensor signal in a post-connected electronic evaluation unit.
In an advantageous embodiment of the invention, the magnetic field sensor element is a magneto-resistive sensor and/or a saturation core probe and/or a Hall element and/or a GMR sensor (Giant Magneto Resistive sensor), whereby the magnetic field sensor element is attached directly onto the wall. Likewise, the magnetic field sensor element can be arranged inside a housing which, in turn, is attached, preferably directly, onto the wall or housing wall; the material of the housing is such that it practically does not influence the magnetic field lines that pass through the housing.
The magnetic field sensor element is capable of adequately detecting the weak leakage field emerging from the housing wall. Of course, the magnetic field of a permanent magnet can also be detected with such an arrangement. A saturation core probe consists of a long coil with a core made of highly permeable material such as, for example, amorphous metal; once the core is magnetically saturated, the impedance of the coil diminishes. A magneto-resistive element is a component made of a magnetically conductive material (permalloy strips), whose resistance changes under the influence of an external magnetic field. GMR sensor elements are a further development of the magneto-resistive sensor element.
Preferably, the housing wall is the wall of a cylinder and the control element is a piston connected to a piston rod, whereby the device for generating the magnetic field is arranged on the piston or else on the piston rod, and said device is a permanent magnet that is connected to one or more pole rings which have faces across from the cylinder wall for feeding the magnetic field into the cylinder wall.
The pole ring or pole rings consist of ferromagnetic material, whereby the permanent magnet can be a magnet ring or can be made up of a plurality of magnets, which is or are arranged in a receiving ring made of a non-magnetizable material, whereby the receiving ring is attached to the piston.
An appropriate electronic evaluation unit for processing the sensor signal and for connecting the magnetic field sensor to an SPS or another peripheral device is post-connected to each sensor element.
In order to better evaluate the leakage flux, the magnetic field sensor can be arranged in a recess of the wall or of the housing wall of the actuating drive.
Moreover, the pole ring consists of soft-magnetic steel and has several permanent magnets, for example, in a cylindrical arrangement, in a receiving ring made of non-magnetizable material. The piston and/or the piston rod of the actuating drive can be made of magnetizable or non-magnetizable material such as, for example, brass. The magnetic field sensor can consist of several spatially differently arranged magnetic field sensor elements for purposes of differential evaluation of the magnetic flux density change and for generating a differential signal. On the piston or on the piston rod, there are pole shoes made of soft-magnetic or ferromagnetic material, between which at least one magnet is held. Likewise, the pole shoes are arranged along a circle and a plurality of magnets are arranged between these pole rings in a cross section plane of the cylinder. The magnets can be arranged in a receiving ring made of non-magnetizable material and preferably be equidistant from each other, whereby the receiving ring is arranged between the ferromagnetic pole rings in such a way that the north or south poles of the magnets are directly across from the pole rings or else touch them.
An essential advantage of the invention lies in the fact that, in contrast to the principle established so far, as described in EP 0,457,762 A (Hall element with flux baffles), due to the specific arrangement of the sensor element and the improvement of the electronics, it is possible to totally dispense with the flux baffles, since the magnetic flux directly enters the sensor element, whose output signal is converted into a corresponding switching signal.
The invention is based on the physical principle that the housing wall of the actuating drive or the cylinder wall has to be made of a ferromagnetic material, whereby a magnetic system has to be attached on the actuating drive, whereby said magnetic system has to generate an adequate magnetic field. When the magnet of the magnetic system of the control element moves past the magnetic field sensor, the magnetically excitable elementary magnets are directed and this is retained as remanence within the housing wall, depending on the material of which it is made. In this process, the elementary magnets, which had been non-ordered until then, are rendered into an ordered state, which forms the remanence field. This generates a more or less weak magnetic leakage field, which emerges from the surface of the housing wall, whose field lines run opposite to the direction of the remanence field within the housing wall. The remaining residual magnetism within the wall or housing wall is directed in accordance with the polarity of the magnetic system of the control element.
The magnetic field sensor mounted on the outside of the housing wall scans the magnetic leakage flux along the housing wall. In doing so, the magnetic field sensor either detects only the field of the remanence or else the polarity, that is to say, the field direction of the remanence.
If the control element approaches the magnetic field sensor, then the strong field of the magnet system generates a leakage field on the outer wall of the housing wall that is superposed over the field of the remanence field; the magnetic field sensor registers this change in the magnetic field.